1. Field of the Invention
The present invention relates to a quadrature amplitude modulation/demodulation device for multi-level digital signals; that is, digital signals having one of a plurality of predetermined levels corresponding to data. The invention more particularly relates to a QAM modulation/demodulation device for carrying out an addition of quadrature components according to a definite functional relationship therebetween, to improve the linearity of modulation and demodulation characteristics.
2. Description of the Related Arts
In general, a method of representing bits or characters by shifting, with respect to a reference, the phase of a carrier wave by an amount corresponding to each type of character, is adopted in a communication system. The transition from one phase state to another is achieved by maintaining a constant amplitude while the signal passes through intermediate phases at an angular rate determined by the phase excursion involved and a shaping function that may be defined. Thus, phase-shift keying (PSK) is angle modulation in which a sinusoidal carrier is switched between two phase states by the transition of a digital data stream. It is actually a digital transmission in which the phase of the carrier is discretely varied in relation either to a reference phase, or to the phase of the previous signal element, in accordance with the data that is to be transmitted.
When four phase states or positions in the time or frequency domains exist within a single period, this is regarded as a quadrature phase-shift keying (QPSK) or a quadrature amplitude modulation (QAM). For instance, a multiple phase shift keying is used in a modulation system in which there are as many phase states of a carrier as there are digital information input code elements to represent.
In general, a balanced mixer with diodes is used as a fundamental circuit for a multi-level modulation/demodulation device, in order to obtain a high frequency modulated wave from a baseband signal or vice versa.
A balanced mixer and a sixteen-level quadrature amplitude modulation (QAM) modulator/demodulator device will be described with reference to FIG. 1. In the figure, numeral 11 denotes a balanced mixer which is composed of balanced transformers 111 and 112, a diode bridge 113 having four diodes, a pair of local carrier input terminals a and a', a pair of baseband input/output terminals A.sub.0 and A.sub.0 ' and a pair of baseband input/output terminals B.sub.0 and B.sub.0 '. When a local carrier input is applied to the balanced mixer 11 from a pair of terminals and a baseband A input is applied to the neutral points N.sub.1, N.sub.2 of the balanced transformers 111, 112 from a pair of input terminals A.sub.0 and A.sub.0 ', then a modulated signal of a carrier frequency is obtained from the output terminals by a modulation operation. On the contrary, if a modulated carrier frequency is applied to a pair of terminals b and b', a demodulation operation is carried out to obtain a baseband output from the terminals Ao and Ao'.
Further, a multi-level modulation/demodulation device will be realized by means of a combination of a plurality of balanced mixers.
FIG. 1 shows an example of a whole constitution of a 16-level quadrature amplitude modulation system modulator/demodulator device.
Numerals 13 and 15 denote hybrid circuits, 11 and 12 balanced mixers, and 14 denotes a 90 degrees phase shifter.
In FIG. 1, a local carrier input is applied from a terminal c and is divided into two parts at a hybrid circuit 13. One part of the input is applied to a balanced mixer 11 through a 90.degree. phase-shifter 14, and the other part is applied directly to a balanced mixer 12. When a baseband input A is applied to the balanced mixer 11 from terminals A.sub.0, A.sub.0 ' and a baseband input B is applied to the balanced mixer 12 from terminals B.sub.0, B.sub.0 ', a multi-level modulation is carried out to obtain a high frequency multi-level modulated signal output composed of 16-level QAM signals at the output terminal d of the hybrid circuit 15, which composes the outputs of both balanced mixers 11 and 12. On the contrary, if a 16-level QAM multi-level modulated wave input is applied to terminal d, a demodulation operation is carried out to obtain a baseband output A and B, respectively, at the terminals A.sub.0, A.sub.0 ' and B.sub.0, B.sub.0 '.
In a multi-level modulation/demodulation device as shown in FIG. 1, if the balanced mixer characteristic is ideal, the amplitude of the baseband signal and the phase of the high frequency multi-level modulated signal are not related to each other, so the amplitude of both signals is completely in a proportional relationship. In reality, it is quite difficult to keep the linearity of such modulation and demodulation characteristics at the high input level.
FIG. 2 is a fundamental diagram of 16-level QAM (4 points every quadrant).
FIG. 3 and FIG. 4 denote an example of modulation and demodulation characteristics in an equivalent mixer. More particularly, FIG. 3 shows amplitude characteristics in the upper diagram and phase characteristics in the lower diagram.
In the figure, the upper diagram shows a baseband input amplitude versus high frequency output characteristics. The lower diagram shows a baseband input amplitude versus phase characteristics. In both diagrams a solid line represents an actual value and a broken line a desired value.
FIG. 4 shows a cos side input versus sine side output characteristics on a vector plane. On account of this characteristic, as the baseband input increases, the phase and amplitude of the high frequency output vector depart from the desired liner characteristic. In FIGS. 3 and 4, if demodulation is desired, the same characteristics will be obtained with regard to demodulation, where the input/output relationship of the baseband signal versus the high frequency signal is conversely illustrated but the characteristics is equivalent.
FIG. 5 is a characteristic diagram of 16-level quadrature amplitude modulator composed of balanced mixers having such non-linear characteristics. If the characteristics of the balanced mixer are ideal or show the desired values, the characteristics of the 16-level QAM modulator/demodulator are represented by the 16 points of the mark (.), but if the balanced mixer having a non-linear characteristic is used, 16-level QAM modulator/demodulator characteristics are represented by the 16 points of another mark (*), and an ideal modulation/demodulation characteristic cannot be obtained.
In the prior art, the level of the baseband signal is lowered to obtain approximately ideal characteristics. But when such a method is adopted not only is the loss in a modulator/demodulator (or modulation/demodulation loss) increased, but also the leakage of local signals cannot be avoided when there is an unbalance in the balanced mixer. As a result the relative value of the output signal level is reduced.